In the marine drilling arena, vessel dynamics have a significant impact on both control and monitoring of the crown block. Although it is not strictly the crown block position with respect to the drill floor that is of consequence, the crown block position is an important consideration. In marine drilling with mobile offshore drilling units (MODU) the top drive may be the primary point of attachment of the drill string to the rig.
Conventionally, in both marine and land drilling, the instrument for measuring block position is a rotary encoder. Various types and attachment configurations of this encoder exist. There are at least two parties on the MODU with an interest in block position, each for slightly different reasons. The drill floor is a primary consumer of the block position information, due to the highly automated nature of drilling systems. The automation system monitors the block position for various control loops and safety interlocks. The other consumer of the block position data is third party service companies on board the MODU, such as mud loggers, measurement while drilling service providers, logging while drilling service providers, and directional drillers.
The encoder's placement on the drill floor has advantages and tradeoffs. The most convenient and reliable location for the encoder is mounted on the shaft of the draw works. The main advantage when mounted on the shaft is that the location allows for easy installation and maintenance. The drawback of this location is that systematic errors may be produced, because the encoder's observation is an indirect measurement. This placement for the encoder measures the drums' current rotation angle. Calibration is necessary to derive the block position. Calibration may be performed by using a direct distance measuring device such as a tape measure or electronic distance measurement (EDM) to generate a look-up table of block position to rotation increment. Placing the encoder on the rotary shaft of the draw works introduces a non-linear systematic error. In addition, the steel wire rope may deform, depending on temperature and load. Yet another possibility is to use a string encoder in place of a rotary encoder.
Conventionally, motion reference units (MRUs) and vertical reference units (VRUs) are used to provide measurements for active compensation for vessel heave. These units may be installed on the drill floor. The outputs from these sensors drive control loop feedback mechanisms such as proportional-integral-derivative (PID) controller loops in the control system in an attempt to maintain a constant weight on the bit.